The invention relates generally to a cassette filter and, more particularly, to a cassette filter composed of a cylindrical case surrounding a dimensionally stable filter pack in a dust-tight manner.
Such a cassette filter is known from U.S. Pat. No. 3,386,231 (the contents of which are incorporated herein by reference). In this patent, both the filter pack and the case are each constructed to be dimensionally stable. While this is an effective approach, it entails significant production costs.
The present invention is directed to the problem of simplifying the production, and hence the production costs, of cassette-type filters. A case, open at its upper and lower ends and formed of flat sheets, surrounds a dimensionally stable filter pack in a dust-tight manner.
The sheets which form the case (the housing of the filter pack) are cemented to one another and to the filter pack by means of adhesive layers, and are spatially fixed in position by the filter pack. The present invention is based on the realization that the dimensional stability of conventional filter packs and the adhesive layers required in any event for affixing the filter pack to the case in a dust-tight manner obviates the need to use a case which itself is dimensionally stable. Consequently, according to the present invention, the case obtains dimensional stability only by being cemented to the filter pack by means of secondary adhesive layers. This, surprisingly, provides good overall stability even though, considered by themselves alone, neither the filter pack nor the case is particularly dimensionally stable. In this manner, the cost of producing a cassette filter according to the invention is considerably reduced.
The sheets needed to form the case can consist of any pliable sheeting, for example wood, metal, cardboard and/or plastic. To minimize weight, the use of so-called webbed double sheets made of plastic has proven advantageous.
The sheets necessary to form the case can be produced independently of one another and be joined to one another and to the filter pack solely by adhesive layers. However, a merging-type construction of the sheets in the form of an endless band is possible as well, and present an advantageous way of commercially producing the cassette filter. In this embodiment, the individual sheets are laid around the filter pack consecutively and separated from the band in the necessary size. The dimensional accuracy and dimensional stability of the cassette filter can be further increased by this means.
In order to prevent damage to the filter pack during transport or installation, the case can be sealed in the area of the air inlet or air outlet side by means of a protective grid or by another sheet, which optionally may be removable.
The adhesive layers can constitute a component of a double-sided adhesive tape. Such tape provides a particularly uniform distribution and thickness, which makes it easier to assure the quality of adhesion. The steps of attaching the elements to one another can be accomplished especially quickly and cleanly. Alternatively, the sheets may be directly coated with an adhesive agent.
An especially reliable degree of sealing and bonding between the individual sheets and the filter pack is achieved by employing for the adhesive layers in all glued areas a compound that consist of an identically compounded thixotropic adhesive, e.g., a polyurethane adhesive. In particular, shifting movements following the joining of the individual layers can be prevented by this means, regardless of position. This is of significant importance with respect to assuring a dust-tight bonding between the filter pack and the sheets, particularly where a change in position and possibly rotation of the cassette filter around a horizontal axis must be carried out prior to the final hardening of the adhesive agent. Such movements may occur, for example, during storage and/or packing.
The attainment of a dust-tight level of bonding between the outer surface of the case and a framework structure receiving the case is substantially simplified if the sheets are provided with a supporting flange pointing outwardly at least at one end, preferably the end facing the oncoming flow of air. The supporting flange extends essentially transversely to the direction of the in-flowing air. This greatly reduces the danger that dust will somehow by-pass the cassette filter. In addition, a sealing element can be provided between the structure receiving the case and the supporting flange. This sealing element can, for example, be a sealing device made of an elastically deformable material. If desired, the sealing element can also be cemented to the supporting flange.
To reduce weight, it has proven advantageous for the sheets and/or the supporting flange to be configured to surround at least one cavity. If needed, the cavity can contain a filler, for instance one made of a rigid foam, in order to improve the dimensional stability of the device. If needed, the filler can be cemented in place within the cavity.
In another embodiment at least two sheets opposite one another are provided with a holding element for a preliminary filter at the end facing the in-flowing air. In the simplest case, this can be formed as a foldable extension of the sheets, which has the further advantage of being unobtrusive during the transport of the cassette filter.